The present invention relates to a copier, facsimile apparatus or similar image forming apparatus.
It is a common practice with an image forming apparatus to develop a latent image formed on an image carrier with a developing device, which includes a plurality of developing rollers. A magnet brush is formed on each of the developing rollers. Japanese Patent Laid-Open Publication No. 10-228179, for example, discloses an image forming apparatus including a developing device in which two developing rollers are arranged in parallel in a direction in which a photoconductive element or image carrier moves. Magnetic poles that effect the hand-over of a developer from one developing roller to the other developing roller are provided with the same polarity. This is successful to prevent the developer from being conveyed through a gap between the developing rollers without being handed over and rendering image density irregular. Let this occurrence be referred to as the entrained movement of the developer for convenience.
The apparatus taught in the above document is more effective when it is operated at a high-speed, because the entrained movement of the developer is aggravated by an increase in the kinetic energy of the developer. Further, the developer stably deposited on the developing rollers in the form of magnet brushes exert scavenging forces that prevent toner from depositing on and contaminating the background of the photoconductive element.
However, the problem with the magnetic poles of the same polarity is that they are apt to lower the density of a halftone image at portions slightly above solid portions or bold characters included in the image, i.e., apt to cause ghosts to appear in such portions. The ghosts are particularly conspicuous in an analog copier that forms the entire uniform halftone image with a small amount of toner.
More specifically, the developing rollers each are rotated at a high speed than the photoconductive element. The developer deposited on the upstream developing roller in the direction of movement of the photoconductive element loses much toner when developing solid portions and characters included in a latent image, which is formed on the photoconductive element. In this condition, the developer is smoothly transferred from the upstream developing roller to the downstream developing roller without being agitated due a repulsive magnetic field formed by the poles of the same polarity between the two rollers. Although this part of the developer transferred to the downstream developing roller again contributes to development, it develops the more upstream portion of the latent image than on the upstream developing roller. This is because the downstream developing roller also rotates at a higher speed than the photoconductive element. Consequently, part of the developer lowered in toner content due to the development of the solid portions and characters lowers the density of the resulting halftone image, causing ghosts to appear in the halftone image.
It is therefore an object of the present invention to provide an image forming apparatus capable of obviating irregular image density and background contamination ascribable to the entrained movement of a developer as well as ghosts apt to appear when a uniform halftone image containing solid portions and bold characters is reproduced.